Don't delay values and errors from async/await functions.

pkg/compiler/lib/src/js_backend/backend.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 part of js_backend;
 
 const VERBOSE_OPTIMIZER_HINTS = false;
 
 class JavaScriptItemCompilationContext extends ItemCompilationContext {
   final Set<HInstruction> boundsChecked = new Set<HInstruction>();
@@ skipping 1879 matching lines @@
     classElement.ensureResolved(compiler);
     return classElement;
   }
 
   Element getSyncStarIterableConstructor() {
     ClassElement classElement = getSyncStarIterable();
     classElement.ensureResolved(compiler);
     return classElement.lookupConstructor("");
   }
 
-  Element getCompleterConstructor() {
+  Element getSyncCompleterConstructor() {
     ClassElement classElement = find(compiler.asyncLibrary, "Completer");
     classElement.ensureResolved(compiler);
-    return classElement.lookupConstructor("");
+    return classElement.lookupConstructor("sync");
   }
 
   Element getASyncStarController() {
     ClassElement classElement = findHelper("AsyncStarStreamController");
     classElement.ensureResolved(compiler);
     return classElement;
   }
 
   Element getASyncStarControllerConstructor() {
     ClassElement classElement = getASyncStarController();
@@ skipping 768 matching lines @@
     String outName = outPath.substring(outPath.lastIndexOf('/') + 1);
     String extension = addExtension ? ".part.js" : "";
     return "${outName}_$name$extension";
   }
 
   void registerAsyncMarker(FunctionElement element,
                            Enqueuer enqueuer,
                            Registry registry) {
     if (element.asyncMarker == AsyncMarker.ASYNC) {
       enqueue(enqueuer, getAsyncHelper(), registry);
-      enqueue(enqueuer, getCompleterConstructor(), registry);
+      enqueue(enqueuer, getSyncCompleterConstructor(), registry);
       enqueue(enqueuer, getStreamIteratorConstructor(), registry);
     } else if (element.asyncMarker == AsyncMarker.SYNC_STAR) {
       ClassElement clsSyncStarIterable = getSyncStarIterable();
       clsSyncStarIterable.ensureResolved(compiler);
       enqueuer.registerInstantiatedType(clsSyncStarIterable.rawType, registry);
       enqueue(enqueuer, getSyncStarIterableConstructor(), registry);
       enqueue(enqueuer, getEndOfIteration(), registry);
       enqueue(enqueuer, getYieldStar(), registry);
       enqueue(enqueuer, getSyncStarUncaughtError(), registry);
     } else if (element.asyncMarker == AsyncMarker.ASYNC_STAR) {
@@ skipping 34 matching lines @@
     AsyncRewriterBase rewriter = null;
     jsAst.Name name = namer.methodPropertyName(element);
     switch (element.asyncMarker) {
       case AsyncMarker.ASYNC:
         rewriter = new AsyncRewriter(
             compiler,
             compiler.currentElement,
             asyncHelper:
                 emitter.staticFunctionAccess(getAsyncHelper()),
             newCompleter: emitter.staticFunctionAccess(
-                getCompleterConstructor()),
+                getSyncCompleterConstructor()),
             safeVariableName: namer.safeVariablePrefixForAsyncRewrite,
             bodyName: namer.deriveAsyncBodyName(name));
         break;
       case AsyncMarker.SYNC_STAR:
         rewriter = new SyncStarRewriter(
             compiler,
             compiler.currentElement,
             endOfIteration: emitter.staticFunctionAccess(
                 getEndOfIteration()),
             newIterable: emitter.staticFunctionAccess(
@@ skipping 311 matching lines @@
   }
 }
 
 /// Records that [constant] is used by the element behind [registry].
 class Dependency {
   final ConstantValue constant;
   final Element annotatedElement;
 
   const Dependency(this.constant, this.annotatedElement);
 }